Microelectronic devices contain many electronic components, such as transistors, capacitors, resistors, etc., within a silicon chip. To form a usable device, a silicon chip requires protection from the environment as well as electrical and mechanical connections to other components. Electronic packaging deals with these requirements. An electronic package containing a silicon chip should provide access to the terminals or bond pads on the silicon chip for input power and signal transmission. The package may also provide the electrical wiring for interconnection among the electronic components on the chip. In addition, the electronic package should protect the circuits on the chip from damage during the chip's service life, and allow heat generated by the operation of the circuits to dissipate.
A typical electronic device package comprises various conducting and insulating materials that have different coefficients of thermal expansions (CTE). For example, an ball grid array (BGA) package may be constructed by attaching a chip to the topside of a substrate with epoxy glue. Bond wires provide connections between the bond pads on the chip pads and bond fingers on the substrate. The chip and the bond wires are embedded within a molding compound, such as a molding epoxy, on the topside of the substrate. Due to the different CTE's of the different materials used for assembling the package, the package may warp seriously during subsequent thermal processes such as a package board mounting reflow process. The warping is more prominent for lead (Pb) free packages because the reflow temperature for Pb free solder is as high as 240° C. to 260° C. The warping of the package may cause problems in solder joints between the package and the board, and consequently the reliability of the device. 